The invention relates to telecommunications, and more particularly, to a digital subscriber line and voice interface between a telephone line and central office equipment of a network operator providing voice and digital subscriber line data services.
A conventional telephone transmission line is typically comprised of a pair of copper conductors that connect a telephone set to the nearest central office, digital loop carrier equipment, remote switching unit or any other equipment serving as the extension of the services provided by the central office. This pair of copper conductors, which is also referred to as a twisted pair, has its leads named as tip and ring. The tip and ring nomenclature is derived from the electrical contacts of an old-style telephone plug. A number of such twisted pairs are generally bundled together within the same cable binder group.
The demand for high bandwidth data transmission over existing telephone transmission lines has led to the development of digital subscriber line (DSL) technology. Several variations of DSL technology (referred to generically as xDSL or simply DSL) are evolving, such as SHDSL (symmetric high-bit-rate DSL), HDSL2 (second-generation high-bit-rate DSL), RADSL (rate adaptive DSL), VDSL (very high-bit-rate DSL), and ADSL (asymmetric DSL). In general, a digital subscriber line is comprised of two DSL modems coupled to one another by a twisted pair. The transmit (Tx) and receive (Rx) signals of DSL communications are carried by the twisted pair.
Some DSL technologies, such as ADSL, have the advantage that voice data transmissions (commonly referred to as Plain Old Telephone Service or POTS) can share the same telephone line with DSL data transmissions. The lower frequency band of the telephone line is used for voice data, while the upper frequency bands are used for digital data. However, because each of these frequency bands operates on the same phone line, such DSL technology employs a splitter to isolate the signals of each band. More specifically, the splitter isolates the low-frequency components (e.g., POTS data) of the transmission, and the high-frequency components (e.g., DSL data) of the transmission. The splitter also operates as a mixer to combine the high-frequency digital data with the low-frequency voice data, and provides the combined signal to the telephone line.
A problem with this splitter approach is that the passive elements (e.g., inductors and capacitors) that are typically used to implement the low and high pass filters of the splitter are bulky and cumbersome. This added bulk requires more physical space to house the splitter, and consequently occupies more physical space in the central office of the telephone company. In the aggregate, the physical space consumption resulting from this approach is costly. Also, this approach is associated with additional material and manufacturing costs involved in building the splitters, as well as costs associated with deploying those splitters.
A silicon broadband subscriber line interface circuit (SLIC), on the other hand, provides an active discrete splitter that is more compact than a passive splitter. However, such technology requires a high-voltage, high-speed silicon process and consumes significantly more power thereby limiting the service circuit line density. Moreover, this approach requires an additional power backup in order to sustain the supported communication channels for the same time period compared to that of a POTS only service in the event of a power failure.
What is needed, therefore, is an improved digital subscriber line and voice interface between a telephone line and central office equipment of a network operator providing voice and digital subscriber line data services
Techniques for interfacing a telephone line and central office equipment of a network operator providing voice and digital subscriber line data services are disclosed. The interface is splitterless in that no conventional bulky splitter is required. A SLIC independent, impedance synthesis network provides compensation for impedance imbalance caused by a capacitor coupled across two-wire interface of voice (e.g., POTS) circuit. The synthesized impedance is frequency variant so that it only affects a specific band of frequencies (e.g., the POTS band frequencies). For frequencies outside that specific band of frequencies, the synthesized impedance is muted so that its effect is essentially disabled. One reason for this frequency variant quality is that the capacitor is desired to be electrically present within the DSL frequency band for purposes of isolating DSL band signals from voice band signals. The features and advantages described in the specification are not all-inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and not to limit the scope of the inventive subject matter.